The Steel Guitar Forum Store 

Post new topic Fender tube amp question:
Reply to topic
Author Topic:  Fender tube amp question:
Ivan Funk

 

From:
Hamburg Pennsylvania, USA
Post  Posted 8 Jun 2023 9:03 am    
Reply with quote

1972 Fender Bassman Ten has a SS rectifier.
3 diodes in series per leg of the HT.

Would there be advantages or disadvantages to using 4 diodes per side or more per side?
I’m trying to drop the B+ a little without resorting to CT zener diode or dropping resistors etc…


View user's profile Send private message Send e-mail

Bill Sinclair


From:
Waynesboro, PA, USA
Post  Posted 8 Jun 2023 10:15 am    
Reply with quote

I don't think a couple more rectifier diodes is going to drop your B+ by more than a couple volts. My 70's Bassman 10 chassis actually has a knockout for a rectifier tube and taped off 5V windings on the transformer. You might see if yours has the same. Switching to a tube rectifier should drop the voltage a bit and also give you a little sag if that's what you're after. If I ever do that to mine, I'll probably just wire it according to a blackface bassman schematic. What's your reason for wanting to lower the B+?
View user's profile Send private message Send e-mail

Ivan Funk

 

From:
Hamburg Pennsylvania, USA
Post  Posted 8 Jun 2023 10:28 am    
Reply with quote

Thanks Bill,

Yes mine is the same as yours - has the 5v windings for a tube rectifier.
Can the PT handle the current draw of a tube rectifier?

Either way - I don't want sag - I like the stiffness of SS.

Reason for wanting to lower B+:
My B+ is 525v with all tubes in and standby off (or closed). I have JJ 6L6gc as power tubes in it. 516v on the plates.

I've been reading forums for the last few days and some people say don't worry about it.

Some attribute it to higher line voltage compared to 40 years ago. I have 375v on the PT secondary HT where the schematic says it should be 365v.

Any ideas for lowering it or should I leave it?

Thanks,
View user's profile Send private message Send e-mail

Raybob Bowman


From:
S. Lake Tahoe, CA, USA
Post  Posted 8 Jun 2023 11:05 am    
Reply with quote

Fender schematics have note votlages are + or - 20 percent. You are fine.
_________________
Sierra U12 4+5 / 1933 Dobro / homemade Tele B-bender
View user's profile Send private message Send e-mail

Dave Grafe


From:
Hudson River Valley NY
Post  Posted 8 Jun 2023 1:16 pm    
Reply with quote

The higher line voltage is a myth. I was there then and 120vac was the norm for most of the USA in 1960.

Your B+ voltage is within Fender spec, and if your amp sounds fine there is no problem, but if it ain't broke we can fix that too.
View user's profile Send private message Send e-mail Visit poster's website

Tim Marcus


From:
San Francisco, CA
Post  Posted 8 Jun 2023 1:30 pm    
Reply with quote

voltage is headroom - let it soar!

the one thing I would check is your heater voltage - it should be 6.3vac and if that drifted high it can shorten tube life and also does make things sound less good than they would at 6.3 or lower. Adding a 1-3ohm 5W resistor after the pilot light will solve that problem for you should you need to solve a problem
_________________
Pedalsteelamp.com
Milkmansound.com
Follow me on Facebook!
Milkman Sound on Twitter
View user's profile Send private message Send e-mail Visit poster's website

Ivan Funk

 

From:
Hamburg Pennsylvania, USA
Post  Posted 8 Jun 2023 7:03 pm    
Reply with quote

Thanks guys.
That's good to hear.
The amp does sound good as it is now.

Hi Tim - yeah the heater voltage is a bit high at 6.9v
Do I add 1-3ohm 5 watt resistor in series with the heater AC line? Like a dropping resistor?

The heater AC already has the two 100 ohm resistors to ground as the schematic shows.

Also - the bias is at 44ma. Is this too high?
Should it be around 36ma?

Would bringing the bias down raise the plate voltage?
Where's a good safe balance?
View user's profile Send private message Send e-mail

Raybob Bowman


From:
S. Lake Tahoe, CA, USA
Post  Posted 8 Jun 2023 7:33 pm    
Reply with quote

6.9v is within the plus/minus 20%. Good.
For bias adjusting, shoot for 70 percent plate dissipation.
https://www.tedweber.com/bias-calc/
_________________
Sierra U12 4+5 / 1933 Dobro / homemade Tele B-bender
View user's profile Send private message Send e-mail

Ivan Funk

 

From:
Hamburg Pennsylvania, USA
Post  Posted 8 Jun 2023 7:56 pm    
Reply with quote

Thanks Raybob.
Very cool.
View user's profile Send private message Send e-mail

Dave Meis


From:
Olympic Peninsula, Washington, USA
Post  Posted 8 Jun 2023 11:24 pm    
Reply with quote

Bias seems a little high at 77% dissipation (525x.044=23.1 watts, 23.1/30=.77).
View user's profile Send private message Send e-mail

Ivan Funk

 

From:
Hamburg Pennsylvania, USA
Post  Posted 8 Jun 2023 11:34 pm    
Reply with quote

Thanks Dave.

New numbers after rebuilding the rectifier board:

523v B+

517v on the plates

42ma bias

What's that 72% dissipation?
Still a bit high?
I'll bring the bias down a bit.
View user's profile Send private message Send e-mail

Ivan Funk

 

From:
Hamburg Pennsylvania, USA
Post  Posted 9 Jun 2023 2:57 am    
Reply with quote

On a whim I re-flowed every solder joint along the heater AC circuit. Now the heater voltage is down to 6.7v
Good enough.

Final numbers:

516v on plates

38ma bias

I'll leave like that. It sounds really good.

Thanks everyone for your help!
View user's profile Send private message Send e-mail

Dave Meis


From:
Olympic Peninsula, Washington, USA
Post  Posted 9 Jun 2023 9:01 am    
Reply with quote

That's a safe 65%. The 70% number is the maximum, not necessarily the target. Under 70%, let your ears be the guide! 😃 My Twin sounds great at 55%-60%. Sounds like you got it figured! 👍
View user's profile Send private message Send e-mail

Ivan Funk

 

From:
Hamburg Pennsylvania, USA
Post  Posted 9 Jun 2023 11:52 am    
Reply with quote

Thanks Dave.

Yes it's nice to have it running safe.

Now to put channel 1 / bass normal channel back to stock.
Someone had "modded" it and the tone controls are backwards and don't do much. Channel 2 sounds really good.
View user's profile Send private message Send e-mail

Henry Brooks

 

From:
Los Gatos, California, USA
Post  Posted 10 Jun 2023 11:12 am    
Reply with quote

The reason why Fender used three diodes in series is because at that time solid state diodes didn't have the very good reverse voltage capabilites. Modern diode are much better so only one diode is needed today. Also, you often find resistors in parallel with the diodes back in the day. The resistors equalize the voltage drop across the diodes.
Henry
View user's profile Send private message Send e-mail

Ivan Funk

 

From:
Hamburg Pennsylvania, USA
Post  Posted 10 Jun 2023 5:21 pm    
Reply with quote

Thanks Henry.
View user's profile Send private message Send e-mail

Bob Sigafoos

 

From:
San Clemente, Calif. , U.S.
Post  Posted 10 Jun 2023 7:13 pm     Bias
Reply with quote

Does the bias pot on this amp serve as a bias balance between the two tubes? Not the bias voltage supply. I've fiddled with one and it seems to be a hum adjustment for the power amp tubes. Which resistor would affect the bias voltage to both tubes?
The 3.3K or the 15K one.
View user's profile Send private message Send e-mail

Ivan Funk

 

From:
Hamburg Pennsylvania, USA
Post  Posted 10 Jun 2023 8:38 pm    
Reply with quote

Yes Bob that is correct - the pot is a bias balance pot.
(1972 Bassman Ten)

The center lug 15k resistor to ground sets the bias.

You can either swap out resistors and check the bias like I did (I ended up at 16k for 38ma bias) or, some people convert the balance pot to work as a bias set pot instead. Or another option is to change the 15k resistor to 10k and add a 25k pot wired to the 10k which goes to ground. Or something like that.

The experts can probably explain it better.

I just noticed that my wall AC voltage is around 120v mid day and goes up to 127v or more at night.
So I've been chasing my tail obsessing over voltages.
I think I need to just go play!
View user's profile Send private message Send e-mail

Pat Chong

 

From:
New Mexico, USA
Post  Posted 13 Jun 2023 12:22 pm    
Reply with quote

Hi Folks,

It is a bias adjustment, not a hum adjustment. If your bias is set too low, the tubes pull too much current and your amp will hum (I'll bet the tube plates glow a little bit, too.)

Anyway, the reason behind the bias adjust is to keep the tubes from pulling too much current while they aren't being used: The less current, the more headroom. If the bias is set too high, though, it could turn the tubes off.

Tube technology is interesting!
.........Pat.
View user's profile Send private message Send e-mail

Bill Sinclair


From:
Waynesboro, PA, USA
Post  Posted 13 Jun 2023 12:45 pm    
Reply with quote

Pat,

On a stock Bassman 10, the pot is a balance adjustment of the bias between two tubes. The way to tell when they are balanced is the point at which there is the least hum.
Fender changed the function of that pot to a balance adjustment on some of their tube amps in the 70's. Perhaps to allow you to use mismatched output pairs? I changed the function of the pot on my Bassman 10 back to a true bias pot with wiring similar to a blackface Bassman.
View user's profile Send private message Send e-mail

Ivan Funk

 

From:
Hamburg Pennsylvania, USA
Post  Posted 13 Jun 2023 2:39 pm    
Reply with quote

Thanks Bill.

Someone had messed up channel 1 so I rebuilt it to a Super Bassman circuit. This brought all voltages down to spec so maybe there had been a short somewhere...?
Sounds really good.
I like the odd-ball deep switch that some people don't like.

There's a very faint minuscule hum at idle with master vol on 10 and channel volumes all the way off. Maybe I only notice it because I recently moved out to the countryside and it's quieter than any place I've ever lived before.
I cleaned up lead dress layout and moved twisted heater wires away form signal wires as much as possible and that helped a little bit. Replaced all caps and resistors in both preamps. The hum is barely noticeable but I have had other Fender tube amps I was able to get dead silent. (Twin, Pro-Reverb, etc..)

If I pull one preamp tube there is less hum.
If I pull both preamp tubes there's is zero hum.
Do some 12ax7 tubes hum more than others?
Should I try different tubes?
Any ideas?


View user's profile Send private message Send e-mail

Pat Chong

 

From:
New Mexico, USA
Post  Posted 13 Jun 2023 2:51 pm    
Reply with quote

Hi Bill,

I did not know it was changed to a "balance" adjust, it would help "mismatched output tubes", as you said.
Being familiar with bias controles and not having seen a "balance" control as such, somehow it looked like a misprint.

Thanks, ..........Pat.
View user's profile Send private message Send e-mail

Dave Grafe


From:
Hudson River Valley NY
Post  Posted 13 Jun 2023 3:21 pm    
Reply with quote

By all means try different preamp tubes, preferably the low-noise 7025 spec over stock 12AX7.
View user's profile Send private message Send e-mail Visit poster's website

Ivan Funk

 

From:
Hamburg Pennsylvania, USA
Post  Posted 13 Jun 2023 10:50 pm    
Reply with quote

Thanks Grafe.
I will do that.
View user's profile Send private message Send e-mail

Ivan Funk

 

From:
Hamburg Pennsylvania, USA
Post  Posted 1 Jul 2023 8:14 pm    
Reply with quote

Final update:

I added a 0.1 ohm 5 watt resistor to the heater line.
Brought it down to 6.4 volts.
Put it back together and played it all day.
Sounds great.
Dead quiet at idle with master vol wide open.

Thanks again everyone for you input!
Very helpful.
A fun rebuild.
View user's profile Send private message Send e-mail


All times are GMT - 8 Hours
Jump to:  

Our Online Catalog
Strings, CDs, instruction,
steel guitars & accessories

www.SteelGuitarShopper.com

Please review our Forum Rules and Policies

Steel Guitar Forum LLC
PO Box 237
Mount Horeb, WI 53572 USA


Click Here to Send a Donation

Email admin@steelguitarforum.com for technical support.


BIAB Styles
Ray Price Shuffles for
Band-in-a-Box

by Jim Baron
HTTP